1. Field
The present invention relates generally to satellite communications and satellite communications systems. More particularly, the present invention relates to estimating and compensating for propagation delays associated with identifying satellite beams in a satellite communications network.
2. Background
Conventional satellite communications systems include one or more terrestrial base stations (hereinafter referred to as gateways), user terminals, such as a mobile phone, and one or more satellites for relaying communications signals between the user terminal and the gateway. The gateway receives signals from and transmits signals to satellites, which could be in orbiting low earth orbit (LEO), processes communication links or calls, and interconnects or transfers calls to and from appropriate ground networks, as desired. That is, the gateway provides an Earth-based link to permit a system user to communicate with other system users or to provide communication links to ground-based service providers, such as publicly-switched telephone networks (PSTNs), data networks, wireless communication systems, or other satellite gateways.
Although mobile phones or wireless user terminals provide users with increased mobility and flexibility, the proliferation of such phones have increased the demands on the associated communications systems. For example, in the case of satellite-based communication systems, determining the position of system users is critically important in establishing communication links with the phone, determining what service provider to use, and providing position location services to the phone user, among other things. However, the flexibility of travel for mobile phones complicates this process.
Most communication satellites project a “footprint” comprising several radio link or communication signal beams grouped to provide coverage for communications with system users in a geographic area covered by the footprint. A particular user terminal may be assigned, albeit temporarily, to use a specific satellite beam for transferring communication signals based upon that user terminal's geographic location. Consequently, a satellite communication system gateway must know the user terminal position in order to provide appropriate communication services to a particular user through an appropriate servicing satellite beam. Thus, knowledge of the specific satellite beam providing service for a particular user or geographical area is basic to the ability of a gateway to provide communication services.
Another aspect of this process is properly establishing a communications link to the other communication services providers, such as the PSTNs and the data networks. These service providers are usually associated with specific Earth-based geographic areas and only process communications links associated with their respective areas. For example, networks may have governmental licenses or various business arrangements with customers to service particular areas. Knowledge of the user terminal position is also needed before these geographically dependent services can be provided. Identification of the satellite beam is a necessary step in determining the position of a user terminal.
A number of conventional approaches are available for determining the position of satellite communication system users. Some techniques, for example, entail measuring a distance between the user terminal and the associated satellites and determining rates of change associated with the determined distance. When these distance measurements are combined with other data, the location of the user terminal may be accurately determined. Techniques for determining user-terminal location using satellite user-terminal range and range rates are disclosed in U.S. Pat. No. 6,078,284, entitled “Passive Position Determination Using Two Low Earth Orbit Satellites;,” U.S. Pat. No. 6,327,534, entitled “Unambiguous Position Determination Using Two Low-Earth Orbit Satellites;” and U.S. Pat. No. 6,107,959 entitled “Position Determination Using One Low-Earth Orbit Satellite.” Furthermore, U.S. Pat. No. 6,137,441, titled “Accurate Range And Range Rate Determination In A Satellite Communications System,” discloses a technique for compensating for satellite motion in order to enhance the accuracy of user-terminal position information.
However, although motion between satellites and associated user terminals can be determined, errors often occur in these measurements due to effects such as antenna gain characteristics or, for example, because a particular satellite may be low in elevation with respect to the user terminal. These errors often result in the gateway misidentifying the servicing or communicating satellite's beam and consequently mis-judging the position of the user terminal. The end result of misidentification of the satellite's beam is often denial of system services, or even complete radio link acquisition failure for the associated user.
Another source of errors may arise as part of the implementation of specific communication techniques or user access methods employed by communication systems to accommodate multiple system users. Numerous techniques exist for providing access to communication systems for multiple system users. Two known multiple-access techniques include time division multiple access (TDMA) and frequency division multiple access (FDMA), the basics of which are well-known in the art. However, spread spectrum modulation techniques, such as code division multiple access (CDMA), are significantly more desirable because of their ability to accommodate multiple users in increasingly bandwidth-limited environments.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307 titled “Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters,” and. U.S. Pat. No. 5,103,459 entitled “System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System,” both of which are assigned to the assignee of the present invention and are incorporated herein by reference. The method for providing CDMA mobile communications was standardized in the United States by the Telecommunications Industry Association in TIA/EIA/IS-95-A, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” referred to herein as IS-95. Other communication systems or techniques are described in the IMT-2000/UM, or International Mobile Telecommunications System 2000/Universal Mobile Telecommunications System, standards covering what are referred to as wideband CDMA (WCDMA), cdma2000 (such as cdma2000 1× or 3× standards, for example) or TD-SCDMA.
In satellite communication systems that use CDMA, a large number of user terminal or phone users, each having a transceiver, communicate through satellites and gateways using spread spectrum communication signals. By using CDMA, the associated frequency spectrum can be reused multiple times, thus permitting an increase in system user capacity. Thus, CDMA is much more spectrally efficient than other user-access techniques. Although CDMA is spectrally efficient, CDMA systems are also somewhat vulnerable to problems associated with the mobile station or terminal mis-positioning errors discussed above. One area where CDMA systems are particularly vulnerable is the area of user handoffs.
A handoff occurs when a mobile station communications session or link, such as an ongoing call or conversation, is passed from one satellite beam to another satellite beam. There are generally two types of handoffs, hard handoffs and soft handoffs. During a hard handoff, when a mobile station moves from within the coverage area of one beam to the coverage area of another, destination or target, beam that is going to provide service, the terminal breaks its communication link with the servicing beam and establishes a new communication link with the destination beam. During a soft handoff, however, the mobile station establishes a link with the destination beam prior to breaking its communication with the current beam. This process is known in the art as make-before-break. Additionally, during a soft handoff, the determination of the proper identification for the destination beam is made in relation to the location of the servicing beam. Thus, during a soft handoff, the mobile station simultaneously communicates with both a servicing beam and a destination beam.
A soft handoff technique is disclosed in U.S. Pat. No. 5,267,261 entitled “Mobile Station-Assisted Soft Handoff in a CDMA Cellular Communications System,” which is assigned to the assignee of the present invention and is incorporated herein by reference. In the system of the '261 patent, the soft handoff process is predicated upon the use of measuring the strength of a pilot signal transmitted by each beam to facilitate access to the satellite by a particular mobile station. By way of background, access to a CDMA-based communication system or communication signals for mobile stations is provided in a forward link, that is, in the direction from the satellite to the mobile station. The forward link includes three types of overhead channels: at least one pilot channel, a synchronization channel, and one or more paging channels. These overhead channels are used by the system to establish and manage communication sessions with the mobile station.
The pilot channel includes transmission of a pilot signal which acts as a beacon for potential system users or subscribers, and is used by user terminals or mobile stations to obtain initial system synchronization and to provide robust time, frequency and phase tracking of the base station transmitted signals. In spread spectrum communication systems such as those based on IS-95, base stations are characterized or distinguished by a phase offset in the pseudorandom-noise (PN) codes used for spreading communication signals, also known as PN offset of the pilot signal. Typically, each terrestrial base-station uses the same spreading code at different code phase offsets. In the alternative, as is more typical of a satellite system, a series of PN codes based on unique PN polynomials are used within the communication system with different PN codes possibly being used for different gateways and for satellites in each orbital plane. It will be readily apparent to those skilled in the art that as many or as few PN codes as desired can be assigned to identify specific signal sources or repeaters in the communication system.
In a satellite based system, in order to determine the proper destination satellite beam, that is which beam is covering the location of the user terminal, from among a number of candidate beams, the user terminal searches for the appropriate pilot signal by determining the pilot signal strength and the PN code or code phase offset. This process is accomplished by performing a correlation operation for each potential code and code phase offset, wherein all received pilot signals are correlated to a particular set of PN code offset values. A method and apparatus for performing correlation operations is described in detail in U.S. Pat. No. 5,805,648, entitled “Method And Apparatus For Performing Search Acquisition In A CDMA Communication System,” which is assigned to the assignee of the present invention and is incorporated herein by reference.
To initially establish a communications link with a communication system, a user terminal must first acquire a pilot signal associated with the system. The user terminal receives the PN code and phase offset information of this pilot signal when it demodulates the pilot signal and system timing by demodulating the synchronization channel. However, before the user terminal is handed off to a new satellite beam, it must correlate newly received pilot signals with the set of PN codes and code offset values to determine the PN offset of the most likely destination satellite beam.
The amount of propagation delay between communication system satellites and the user terminal is often significant and uncertain, and may cause unknown shifts in PN offset values being detected. That is, the user terminal detects a larger phase shift due to the delay rather than the originating signal source. These shifts may result in the user terminal misidentifying a new hand-off or destination satellite beam. Unknown propagation delays are especially probable when the new destination satellite is low on the horizon with respect to the current servicing satellite in the cited references.
What is needed, therefore, is a technique to compensate for the effects of propagation delays by permitting the user terminal to independently verify PN offset measurements associated with destination or new target satellites in a manner that estimates propagation delays.